[PATCH v2] selftests/mm: khugepaged: initialize file contents via mmap

[Vineet Agarwal <agarwal.vineet2006@gmail.com>]

file_setup_area() currently allocates anonymous memory, fills it,
and writes it into the backing file used for collapse testing.

Instead of copying data through write(), map the file directly with
MAP_SHARED after resizing it with ftruncate(), initialize the mapped
area in place, and sync it with msync().

This simplifies the setup path and avoids the need for explicit
partial write handling.

Signed-off-by: Vineet Agarwal <agarwal.vineet2006@gmail.com>
---
 tools/testing/selftests/mm/khugepaged.c | 25 +++++++++++++++++++++----
 1 file changed, 21 insertions(+), 4 deletions(-)

diff --git a/tools/testing/selftests/mm/khugepaged.c b/tools/testing/selftests/mm/khugepaged.c
index 3fe7ef04ac62..f6e171e1da4f 100644
--- a/tools/testing/selftests/mm/khugepaged.c
+++ b/tools/testing/selftests/mm/khugepaged.c
@@ -369,7 +369,6 @@ static void *file_setup_area(int nr_hpages)
 	int fd;
 	void *p;
 	unsigned long size;
-
 	unlink(finfo.path);  /* Cleanup from previous failed tests */
 	printf("Creating %s for collapse%s...", finfo.path,
 	       finfo.type == VMA_SHMEM ? " (tmpfs)" : "");
@@ -381,11 +380,29 @@ static void *file_setup_area(int nr_hpages)
 	}
 
 	size = nr_hpages * hpage_pmd_size;
-	p = alloc_mapping(nr_hpages);
+	if (ftruncate(fd, size)) {
+		perror("ftruncate()");
+		close(fd);
+		exit(EXIT_FAILURE);
+	}
+	p = mmap(BASE_ADDR, size, PROT_READ | PROT_WRITE,
+		MAP_SHARED, fd, 0);
+	if (p == MAP_FAILED || p != BASE_ADDR) {
+		perror("mmap()");
+		close(fd);
+		exit(EXIT_FAILURE);
+	}
 	fill_memory(p, 0, size);
-	write(fd, p, size);
-	close(fd);
+
+	if (msync(p, size, MS_SYNC)) {
+		perror("msync()");
+		munmap(p, size);
+		close(fd);
+		exit(EXIT_FAILURE);
+	}
+
 	munmap(p, size);
+	close(fd);
 	success("OK");
 
 	printf("Opening %s read only for collapse...", finfo.path);
-- 
2.54.0